Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long‐term enhancement of forest productivity under elevated CO 2

Ecology Letters (2011) 14: 349–357 Abstract The earth’s future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO 2 . Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C‐cascade through the root‐microbe‐soil system; increases in the flux of C belowground under elevated CO 2 stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO 2 as a result of increases in canopy N content and higher photosynthetic N‐use efficiency. The ecosystem‐level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.